The present invention relates to electrical resistance tomography, particularly to imaging the electrical resistivity distribution in a formation remote from a borehole, and more particularly to an electrical resistance tomography method which combines making electrical resistance measurements inside a steel casing in a borehole and electrical resistance measurements made from an uncased borehole or a borehole not cased with an electrically conducting casing, and thereafter tomographic inversion of the electrical resistance measurements.
Electrical resistivity tomography (ERT) is a method for determining the electrical resistivity distribution in a volume from discrete measurements of current and voltage made within the volume or on its surface. A model of the subsurface of interest is produced by obtaining electrical resistivity measurements from various points of the subsurface area of interest and the thus generated data is used in an inverse mathematical calculation to produce the model.
In recent years, there has been a growing interest in measuring formation resistivity through metal cased wells. There are tens of thousands of cased wells where the resistivity logs are missing or obsolete due to changes of the subsurface conditions and cannot be logged by conventional open-hole methods. The ability to obtain resistivity information through cased wells can reaccess existing reservoirs for effective recovery of oil and gas, or geothermal energy, without the cost and time of drilling new wells. This technique can also monitor resistivity changes over time associated with subsurface flow processes such as injection or leakage of contaminants from a waste site, steam or water flooding operations or enhanced oil recovery, or the process of geothermal production.
Various different approaches have been proposed or utilized for obtaining the electrical measurement data. For example, U.S. Pat. Nos. 4,796,186 issued to A. A. Haufman; No. 4,820,989 issued to W. B. Vail, III; No. 4,837,518 issued to M. F. Gard et al.; and No. 4,882,542 issued to W. B. Vail, III involve measuring formation resistivity with direct current (DC) devices that operate within a cased well. W. B. Vail et al. have developed a tool, called Through-Casing Resistivity Tool, that produces excellent results, see Vail et al., "Formation resistivity measurements through metal casings", Transactions, SPWLA 34th Annual Logging Symposium, Jun. 13-16, 1993, Calgary, Canada. This tool uses a multielectrode configuration to determine the casing conductance and the contact resistance. The contact resistance is the resistance offered to current leaving the metal casing and flowing radially into the formation and is proportional to the formation resistivity. Other examples of prior efforts are provided by W. Daily et al., "Cross-borehole resistivity tomography", Geophysics, Vol. 56, No. 8 (August 1991), pgs. 1228-1235; and W. Daily et al., "Electrical Resistivity Tomography of Vadose Water Movement", Water Resources Research, Vol. 28, No. 5, pgs. 1429-1442, May 1992.
The above-referenced patented approaches were directed to the single hole logging problem where the sampled radius is limited to the regions near the well. The use of crosshole measurements can determine the electrical properties between wells. However, presently used DC crosshole methods require open wells or installing electrodes outside non-conductive casings. Metal cased well were not used in the above-referenced patented approaches. However, Schenkel and Morrison, "Electrical resistivity measurement through metal casings", Geophysics, 59, pgs 1072-1082, 1994, indicate that a metal cased well may be also used effectively in crosshole surveys.
The present invention involves the next step and uses the cased well in crosshole measurements and makes electrical resistance measurements inside a steel casing of a borehole and from a borehole not cased with an electrically conducting casing to determine the electrical resistivity distribution between the boreholes.